3GPP Long Term Evolution (LTE) is a standard for mobile phone network technology. LTE is a set of enhancements to the Universal Mobile Telecommunications System (UMTS), and is a technology for realizing high-speed packet-based communication that can reach high data rates on both downlink and uplink channels. As illustrated in FIG. 1, LTE transmissions are sent from base stations 102, such as Node Bs (NBs) and evolved Node Bs (eNBs) in a telecommunication network 106, to mobile stations 104 (e.g., user equipment (UE)). Examples of wireless UE communication devices include mobile telephones, personal digital assistants, electronic readers, portable electronic tablets, personal computers, and laptop computers.
The LTE standard is primarily based on Orthogonal Frequency Division Multiplexing (OFDM) in the downlink, which splits the signal into multiple parallel sub-carriers in frequency, and Single Carrier Frequency Domain Multiple Access (SC-FDMA) in the uplink. A transmit time interval (TTI) is the basic logical unit. A radio resource element (RE) is the smallest addressable location within a TTI, corresponding to a certain time location and a certain frequency location. For instance, as illustrated in FIG. 2, a sub-frame 200 comprised of REs may be transmitted in a TTI in accordance with the LTE standard, and may consists of sub-carriers 204 in the frequency domain. In the time domain, the sub-frame may be divided into a number of OFDM (or SC-FDMA) symbols 208. An OFDM (or SC-FDMA) symbol 208 may include a cyclic prefix 206. Thus, the unit of one sub-carrier and one symbol is a resource unit or element 202.
Wireless communication systems may be deployed in a number of configurations, such as Multiple-Input, Multiple-Output (MIMO) radio systems. An exemplary MIMO system including a base station 302, such as an eNB, and user equipment 304 is shown in FIG. 3. When a signal is transmitted by the eNB 302 in a downlink, i.e., the link carrying transmissions from the eNB to the UE 304, a sub-frame may be transmitted from multiple antennas 306,308 and the signal may be received at a UE 304, which has one or more antennas. The radio channel distorts the transmitted signals from the multiple antenna ports. UE 304 may use receiver-diversity signal processing schemes to improve performance.
In an LTE system, transmissions intended for a first user are often overheard by a second, unintended user. The second user may utilize overheard data packets in various ways. For instance, “Completely Stale Transmitter Channel State Information is Still Very Useful,” by M. Maddah-Ali and D. Tse, Allerton Conference, 2010, describes a multi-user downlink MIMO scheme with a mechanism for information exchange between single antenna terminals, where the terminals feed back channel state information (CSI) to the serving base station. The serving base station exploits this CSI to broadcast an additional signal, which each terminal uses to create a virtual diversity receiver (VDR). This type of information exchange may be referred to as “stale feedback,” since the channel may have changed significantly by the time the base station transmits the extra signal. In this scheme, a mobile device that receives signals on only a single antenna may still take advantage of simple receive-diversity processing techniques. Similarly, “Multi-User ARQ,” by Peter Larsson and Nicklas Johansson, IEEE VTC Spring, 2006, which is incorporated by reference herein in its entirety, discusses an Automated Repeat request (ARQ) control scheme that exploits the fact that users frequently overhear each other's information.
Other techniques utilize an explicit pilot structure that can be effectively used to facilitate the estimation of channel parameters at the receivers, including true channel taps, as well as estimations of the virtual channels created by the VDR scheme. However, the presently know schemes do not address the number of information bits per TTI or transmission rate that may be supported in a given scheme.
The implemented transmission rate for a given scheme is dependent on what information is available regarding the quality of transmissions received at a user device. Two exemplary types or rate control mechanisms are a “fast rate control” mechanism, based on instantaneous channel conditions, and a “slow rate control” mechanism, based on average channel conditions. The two mechanisms may be best suited for different scenarios. For instance, at low Doppler speeds, channel prediction is accurate, such that fast rate control would be preferable. However, at higher Doppler speeds, it may be preferable to implement slow rate control in order to avoid prediction errors and ensure that the selected rate matches the average channel state. A communication system may also include additional mechanisms to improve robustness, such as an outer-loop control mechanism to adjust certain estimates if previous iterations result in a rate that is too high or too low. For instance, outer-loop control may be based on monitoring the number of HARQ transmissions actually required compared to a target value.
In a fast rate control scenario, a user device (or the base station) may use the most recent channel estimate, Hij[t], between a transmit antenna j and receive antenna i with a noise estimate, zi[t], to derive a desired transmission rate for information transmitted between antennas i and j. For instance, letHj[t]=[H1j[t],H2j[t]]T  (i)be the channel coefficient vector associated with transmit antenna j at time t, with each of the elements corresponding to one receive antenna. In equation (i), the superscript “T” represents the transpose of a vector or matrix.
Using a linear minimum mean-square error (MMSE) receiver, the transmission rate of the data stream from a first transmit antenna could be determined based on the signal to noise plus interference ratio (SINR)SINR1=P1H1H[t](P2H2[t]H2H[t]+Rz)−1H1[t]  (ii)where the superscript “H” represents the conjugate transpose, Pj is the transmit power from antenna j, or the power adjustment factor for transmit antenna j, and Rz is the covariance of noise,z[t]=[z1[t],z2[t]]T  (iii),which can be estimated using reference symbols.
Similarly, the SINR of a data stream from a second transmit antenna can be determined bySINR2=P2H2H[t](P1H1[t]H1H[t]+Rz)−1H2[t]  (iv).If a successive interference cancellation (SIC) receiver is used, the data stream from one transmit antenna is detected and cancelled before detecting the other data stream. The order of detection may be fixed, or may be based on some additional criterion. Without loss of generality, we describe the case where the data stream from the first antenna is detected and canceled first. Then SINR1 is unchanged, and SINR2 may be estimated instead using:SINR2=P2H2H[t]Rz−1H2[t]  (v)The case where the stream from the second antenna is detected and cancelled first is handled similarly. Regardless of receiver type, SINR1 and SINR2 may be translated into transmission rates using a look-up table, for example, as shown in FIG. 8. In this particular example, transmission rate is determined based on the combination of modulation and coding rate, together.
In a situation where instantaneous or recently updated channel coefficients are not available or reliable, slow rate control may be used because it is based on longer-term statistics. For instance, a receiver may estimate the power of one or more channel taps, using a time average, determined by
                              P                                    H              ij                        ⁡                          [              t              ]                                      =                              1            K                    ⁢                                    ∑                              k                =                0                                            K                -                1                                      ⁢                                                                                                H                    ij                                    ⁡                                      [                                          t                      -                      D                      -                      k                                        ]                                                                              2                                                          (        vi        )            where D is delay, and K is a number of values. Similarly, a noise power estimate may be determined by
                              P                                    z              i                        ⁡                          [              t              ]                                      =                              1            K                    ⁢                                    ∑                              k                =                0                                            K                -                1                                      ⁢                                                                                                z                    i                                    ⁡                                      [                                          t                      -                      D                      -                      k                                        ]                                                                              2                                                          (        vii        )            for the same or different values of D and K. Alternative averaging methods may be suitable as well. Assuming that the power estimates are available to a MMSE receiver, the SINR of a data stream from the first transmit antenna would be determined by
                              SINR          1                =                                                            P                1                            ⁢                              P                                                      H                    11                                    ⁡                                      [                    t                    ]                                                                                                                        P                  2                                ⁢                                  P                                                            H                      12                                        ⁡                                          [                      t                      ]                                                                                  +                                                P                                      z                    -                    1                                                  ⁡                                  [                  t                  ]                                                              +                                                    P                1                            ⁢                              P                                                      H                    21                                    ⁡                                      [                    t                    ]                                                                                                                        P                  2                                ⁢                                  P                                                            H                      22                                        ⁡                                          [                      t                      ]                                                                                  +                              P                                                      z                    2                                    ⁡                                      [                    t                    ]                                                                                                          (        viii        )            and the SINR of a data stream from the second transmit antenna would be determined by
                              SINR          2                =                                                            P                2                            ⁢                              P                                                      H                    12                                    ⁡                                      [                    t                    ]                                                                                                                        P                  1                                ⁢                                  P                                                            H                      11                                        ⁡                                          [                      t                      ]                                                                                  +                              P                                                      z                    1                                    ⁡                                      [                    t                    ]                                                                                +                                                                      P                  2                                ⁢                                  P                                                            H                      22                                        ⁡                                          [                      t                      ]                                                                                                                                        P                    1                                    ⁢                                      P                                                                  H                        21                                            ⁡                                              [                        t                        ]                                                                                            +                                  P                                                            z                      2                                        ⁡                                          [                      t                      ]                                                                                            .                                              (        ix        )            If the receiver were a SIC receiver, and the first stream is detected and cancelled first, then SINR1 would be unchanged, and SINR2 would become:
                              SINR          2                =                                                            P                2                            ⁢                              P                                                      H                    12                                    ⁡                                      [                    t                    ]                                                                                      P                                                z                  1                                ⁡                                  [                  t                  ]                                                              +                                                                      P                  2                                ⁢                                  P                                                            H                      22                                        ⁡                                          [                      t                      ]                                                                                                  P                                                      z                    2                                    ⁡                                      [                    t                    ]                                                                        .                                              (        x        )            The case where the second stream is detected and cancelled first is handled similarly. Again, each SINR could be converted to a transmission rate through the use of a look-up table.
Despite the foregoing, there remains a need for methods and systems for determining transmission rates based on virtual data in order to fully realize the improved transmission properties available in a virtual diversity scheme. A stale feedback scenario, for instance as described in Completely Stale Transmitter Channel State Information is Still Very Useful, by M. Maddah-Ali and D. Tse, cannot be effectively utilized without a rate control mechanism.